~anthropic/claude-sonnet-latest → A (6:4)

Side A fixes a real security bug (fail-open voting fallback to anon) with a fail-closed rewrite, adds Secure cookie support, gates mock OAuth behind an env flag, hardens open-redirect sanitization, and adds unit tests plus nav UI improvements—concrete, security-relevant lasting value. Side B is a larger refactor (new pair.rs module, vote.rs rewrite, ItemId::from_storage normalization) that is useful but is mostly internal restructuring/feature polish without addressing a correctness or security defect, and is labeled just 'refactor' with less durable architectural necessity than A's fixes.